The present invention relates to an electroproportional solenoid valve unit having a valve housing in which a piston is slidably arranged so as to be displacable by the push rod of a solenoid. The piston is in the form of a hollow piston and on either end of the piston a hydraulic chamber is provided. The piston also has two annular channels that can be connected to hydraulic connectors of the valve housing.
When the solenoid is excited in this known valve device, the piston in the form of a hollow piston is displaced against the force of a pressure spring. This action leads to the opening of one of the hydraulic connectors to the consuming device to be activated so that the hydraulic medium can flow via this open hydraulic connector to the consuming device. The hydraulic medium being displaced from this consuming device is transferred to the other hydraulic connector and flows from there into the hollow interior of the piston. From there the hydraulic medium flows back into the reservoir. At the end face of the piston which is facing the solenoid a hydraulic pressure is generated in the hydraulic chamber as a function of the volume of the hydraulic medium flowing through the hollow interior of the piston limited by its diameter. The hydraulic pressure thus generated is greater than the pressure which acts on the opposite end face of the piston. Accordingly, with increasing amperage, with which the solenoid loads the corresponding control edges, the position of the piston and the flow of the hydraulic medium is changed. As soon as a critical throughflow value is passed, a sudden change of the piston position occurs due to the pressure forces which act on the end face of the piston facing the solenoid and add to the magnetic force. Below this critical throughflow value the throughflow/amperage characteristic line still follows the desired course. Above this critical value however a sudden undesired change of the flow volume occurs.
It is therefore an object of the present invention to improve an electroproportional solenoid valve unit of the aforementioned kind such that the throughflow volume of the hydraulic medium changes continuously as a function of the amperage with which the solenoid is loaded.